A capacitance video recording and playback system has been disclosed in Clemens U.S. Pat. No. 3,842,194, incorporated herein by reference. The disc described therein comprises a plastic disc containing an information track having audio, video and color information in the form of geometric variations in the disc surface. The geometric variations in said track correspond to capacitance variations representative of video signals which are picked up by a stylus riding along the track and converted into video information suitable for display by a television receiver. The video disc of Clemens had a conductive metal coating to provide the conductivity required for capacitive pickup and a thin layer of a dielectric material thereover. An electrode on the playback stylus completed the capacitor.
Improvements have been made in this system whereby the disc is molded from a conductive plastic composition, as disclosed by Fox et al. in copending application Ser. No. 105,550, filed Dec. 20, 1979, which is a continuation of application Ser. No. 818,279, filed July 25, 1977, now abandoned. This composition is prepared by blending a sufficient amount of finely particulate conductive particles, for example carbon black, with a plastic material, such as polyvinylchloride, so that the resulting composition has the conductivity required for capacitive playback. A thin layer of the plastic material surrounds each of the conductive particles thereby providing a thin dielectric layer on the surface of the molded disc.
The use of a conductive plastic composition of the type described by Fox et al. eliminates the need for separate metal and dielectric layers on the disc surface and thus has simplified manufacture of the disc. There is, however, the need to have the layer of resin covering the conductive particles be virtually a total covering since it has been found that loss of the dielectric layer at the surface of the molded disc is a major cause of playback defects of the finished video disc. A means of accurately determining whether the resin coating of conductive particles meets minimum product requirements would be of obvious economic benefit in a number of particulars.
First, if it were determined that a given batch of conductive particles dispersed in a resin would not meet product requirements, deterioration of costly stampers during molding of the discs therefrom would be prevented. Second, accurate measurement of the disc after molding to cull out those which would not meet minimum playback requirements would result in a savings in processing operations, packaging materials and the like. Finally, an accurate means of determining early in the manufacturing process whether video discs prepared from a given batch of formulation will meet minimum playback specifications with regard to resin coverage of the conductive particles is advantageous in reducing the number of discs which do not meet quality specifications. Such a determination is provided in accordance with this invention.